CURRENT THEORIES:
-It's kleiner with an ammo belt (HL2: Episode 3).
-It's a new source engine for a new Half-Life game (HL3).
-It's Duke Nukem Forever, now appropriated by Valve.
-It's fake.

Okay, here's the situation, maggots.

At 2050 Greenwich Meridian Time today valve apparently added an image to the frontpage of their website. As far as the internet has been able to gather so far, the image was removed precisely four minutes afterwards. This was long enough for someone from 4chan to grab a screenshot, but nothing more.

Exhibit A - The Screenshot:

There was a short-lived thread full of manic speculation, the only remnant of which was some MSpaint highlighting.

Exhibit B - The Paint:

Gentlemen, you may remember the dicking around on Valve's part that we were subject to during the Sniper/Spy update a few months back. It seems Gabe is back to his old tricks. Ready your tinfoil hats, and let the rumour mill grind into action.

Edit:
Exhibit C - The Brightened Screenshot courtesy of Mike55:

Stay back jimbomcb, we have 400+ readers who will crush you under boxes.

The Steam forums are absolutely unaware of this, which I can't believe in the event the image actually appeared.

The only thing that could potentially top a semi-secret announcement of Episode 3, would be something like Half-Life 3 or Opposing Force 2, and even that is secondary to the possibility of Duke Nukem Forever being developed and released by Valve.

The Steam forums are absolutely unaware of this, which I can't believe in the event the image actually appeared.

The only thing that could potentially top a semi-secret announcement of Episode 3, would be something like Half-Life 3 or Opposing Force 2, and even that is secondary to the possibility of Duke Nukem Forever being developed and released by Valve.

There's wishful thinking, and then there's sheer naivety.

The fact remains that people claim to have seen it. Besides, steampowered.com users aren't much more likely to visit the valvesoftware.com frontpage than 4chan users.